Most printed circuit boards are produced by first coating a printed circuit board substrate, having a solid copper cladding, with a photoresist material. When selectively exposed and developed, part of the photoresist is removed and the remaining photoresist forms a pattern corresponding to the light and dark areas of the illumination. After formation of the developed photoresist pattern, the copper cladding is typically subjected to an etching step in which that part of the copper cladding that is not coated by the photoresist is removed.
In the main, one of two methods of selectively exposing the photoresist is used. One method utilizes a master, such as a film, on which the required pattern (or its inverse) is formed. The photoresist coating is exposed to strong illumination, through this film. The exposed board is then developed and etched as described above. For highly precise patterns with small object sizes, this method has a number of significant drawbacks. Firstly, the films may stretch or otherwise distort. This stretching causes sometimes significant variations between the sizes and positions of the patterns formed on various layers of a multilayer or double sided board. Second, the wear and tear on the films requires frequent changing the films. Third, any changes in the layout require a new set of films and often several new sets of films. Finally, it is difficult to compensate for small variations in sizing that occur during processing of the boards.
A second method, which is the subject of the present invention, utilizes a scanning laser beam to scan the photoresist coating to write the pattern thereon. This method is known as the “direct writing” method. In principle, direct writing overcomes many of the drawbacks of the prior art. In practice, conventional direct writing methods often have problems of their own. In particular, conventional direct writing systems are generally much slower than the film exposure methods and require much higher capital expenditure. Although, in principle, the accuracy and resolution of direct writing is high, many practical considerations, such as the ability to speedily deliver high energy radiation to expose the photoresists, have limited the throughput as well as the actual accuracy and precision of boards produced by conventional direct writing methods.
It is known in the art to scan a printed circuit board with a plurality of beams to increase the scanning speed of direct writing systems. In general such multi-beam scanning is utilized in a number of fields such as in the preparation of reticules for the printed circuit industry and in electrophotographic printing machines. However, in such applications the power requirements are low and the size of the objects scanned is generally relatively small.
U.S. Pat. No. 5,635,976 describes a system for improved feature definition in producing reticules. In this system the reticule is scanned utilizing a beam address at a resolution at least four times as high as the object pixel resolution. A single laser scan line or multiple scan lines are disclosed.
U.S. Pat. No. 5,495,279 describes a system for exposure of patterns for very large device. In this device an oval laser beam is used. The beam is segmented into at least 100 parallel segments, each of which is separately addressable, such that 100 lines of pixels may be written together.
A further requirement of direct writing systems is that the system know exactly where on the written surface the beam is situated at any time. One way of making this determination is to boresight a test beam with the writing beam. The test beam is separated from the writing beam prior to the impingement of the writing beam on the surface of the printed circuit board. A scale selectively reflects a portion of the test beam. The reflected beam is then detected and the detected signal is used as the basis for a data clock. However, in order to provide for ease of boresighting and especially for separation of the boresighted beams, the beams are of different wavelengths. Unfortunately, beams of different wavelengths are affected differently by the optics and do not track perfectly. Furthermore, only a portion of the relevant path is traversed by both beams and the f-θ lens typically used in such scanners is not fully traversed by both beams. One such system is the LIS DirectPrint 40 of Laser Imaging Systems GmbH of Jena, Germany.
Direct writing systems generally operate with printed circuit boards of various thicknesses. While optical focusing of light is well known, it is not used for direct writing systems due to the complexity of the optics. Therefore, prior art systems utilize a table whose height is variable. For different heights of board, the height of the table is adjusted such that the surface of the board is in focus at a predetermined focal plane. However, especially for large boards, such a table is mechanically complicated, especially if other mechanical adjustments, such as angle of the board with respect to the table, must be made.
Rasterized laser scanning systems typically suffer from inaccuracies caused by the polygon used to scan the beam. Wobble cause random errors in the writing of the laser beam in the cross-scan direction. Imperfections in the scanner optics causes other, usually systematic errors.
Generally, it is important to precisely position the optical exposure of the photoresist (and the subsequent etched pattern) on the board. While this is not very important for single sided inner layers of composite boards for outer layers formed on already laminated boards and for double sided boards, in which holes are drilled to connect features within the board or on opposite sides of the board or layer, the exact placement of the patterns is imperative. This can be achieved by referencing the scan data to predetermined features, for example for inner layers, referencing to features that appear on both sides of the board. One such normally used feature is a drilled hole. In general, an unwritten board has one or more drilled holes present in the board, which holes mate with pins on the scanner. For double sided inner layers, scanning is thus referenced to the holes for both sides of the board.
However, such a system is not wholly satisfactory. The accuracy possible with pin alignment is relatively limited, as compared with the required printed circuit element resolutions. Attempts to reduce the tolerances of the holes to the pins results in damage to the holes and subsequent poor alignment.